Polarimetric scatterometer for critical dimension measurements of periodic structures

ABSTRACT

An optical measurement system for evaluating a sample has a azimuthally rotatable measurement head. A motor-driven rotating mechanism is coupled to the measurement head to allow the optics to rotate with respect to the sample. In particular, a preferred embodiment is a polarimetric scatterometer (FIG.  1 ) for measuring optical properties of a periodic structure on a wafer sample ( 12 ). This scatterometer has optics ( 30 ) directing a polarized illumination beam at non-normal incidence onto the periodic structure. In addition to a polarizer ( 8 ), the illumination path can also be provided with an E-O modulator for modulating the polarization. The measurement head optics also collect light reflected from the periodic structure and feed that light to a spectrometer ( 17 ) for measurement. A polarization beamsplitter ( 18 ) is provided in the collection path so that both S and P polarization from the sample can be separately measured. The entire measurement head can be mounted for rotation of the plane of incidence to different azimuthal directions relative to the periodic structures on the wafer. The instrument can be integrated within a wafer process tool in which wafers may be provided at arbitrary orientation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) from prior U.S.provisional application No. 60/280,714, filed Mar. 30, 2001.

TECHNICAL FIELD

The present invention relates to optical measurement instruments, suchas scatterometers, reflectometers and ellipsometers, for wafer testingduring the manufacture of integrated circuits.

BACKGROUND ART

In the manufacture of integrated circuits, very thin lines or holes downto 100 nm or sometimes smaller are patterned into photo resist and thenoften transferred using an etching process into a layer of materialbelow on a silicon wafer. It is extremely important to inspect andcontrol the width and profile (also known as critical dimensions or CDs)of these lines or holes. Traditionally the inspection of CDs that aresmaller than the wavelength of visible light has been done using largeand expensive scanning electron microscopes. In many cases, however,manufacturers would like to have measurements immediately after thephotoresist has been patterned or etched to have tight control of theprocess before it drifts out of spec. Testing the wafer early duringproduction and controlling the fabrication steps according to the testresults helps to keep production costs low and to keep yields high.Ideally the measurement tool would be integrated into the wafer trackthat develops the photoresist or integrated into the wafer etching tool.

In typical stand-alone instruments, the wafer is moved on a stage, whilethe measurement optics remain stationary. Also, when the angle ofincidence on the wafer is other than zero (e.g. in ellipsometers), thewafer is preferably oriented so that the plane of incidence isperpendicular to the lines on the wafer.

An integrated CD measurement tool must be both fast and compact, andmust not damage the wafer under test. The size constraints usually meanthat the wafer can not be translated across its full diameter in 2horizontal axes to measure different sites on the wafer. Hence, aportion of the test instrument must move in one or more axes to coverthe wafer. The wafer might also rotate, but this is less desirable insystems without full X-Y movement that have a preferred measurementorientation with respect to certain wafer features. Furthermore, somewafer processing tools into which the present invention may beintegrated require that the wafer not move so that the processing toolrobot can pick up the wafer at any time. The wafer may also be loadedinto the measurement tool at an arbitrary angle creating furthercomplications for instruments that have a preferred measurementorientation with respect to certain wafer features.

One general technique that has promise for integrated CD measurements isscatterometry. This technique takes advantage of the fact that an arrayof small lines or holes affect the properties of the light in the zeroorder that is reflected (or, for transparent samples, transmitted) fromsuch an array. Various measurable properties of the zero-order lightwill vary depending on the dimensions of the structure on the wafer.Often such parameters are measured versus wavelength, and in some cases,versus angle of incidence on the sample. Normal-incidence spectroscopicreflectometers show particular promise because they can be used with thewafers in any arbitrary orientation.

Typically, CD measurements have been made using instruments such asellipsometers or reflectometers that were originally designed to measurefilm thickness. The data from such instruments is usually fed to aprocessor, which analyzes the measurements, usually by accessing alibrary of theoretically generated data for a range of array dimensionsand film properties near those of the expected dimensions of the sample.The measured data are compared to the library and a best fit match to adata set in the library is found. The processor then outputs thecorresponding dimensions.

Since there are multiple independent unknown variables that may need tobe measured, such as line width, line edge slope, top film thickness,underlying film thickness, or film refractive index, it is desirablethat the measurement technique measure as many multiple independentparameters as is practical.

Coulombe et al. ('Ellipsometric-Scatterometry for sub-0.1 μm CDmeasurements, 'SPIE, Vol. 3332, p. 282-293) investigated reflectometryand ellipsometry of line gratings as a function of angle of incidenceand azimuth.

One object of the present invention was to create a scatterometer formeasuring CDs and possibly overlay error on periodic structures that iscompact and well suited for integration into a wafer process tool.

Another object was to be able to measure on structures at differentazimuth orientations.

Another object was to be able to collect as much independent data aspractical from the sample. Another object was to be able to measurestructures 100 nm wide or smaller.

Another object of the invention was to be able to measure structures atthe optimal azimuth angle or angles regardless of the azimuthorientation of the sample.

SUMMARY OF THE INVENTION

These objects are met by a polarimetric scatterometry instrument thatoptically measures properties of periodic structures on a sample, usingpolarized light incident upon such structures. The polarized light isincident on samples at non-normal incidence (defined here as greaterthan 4° from perpendicular to the sample surface), and the reflectedlight is collected, fed into a spectrometer, and the measurements usedto determine the width, profile or thickness of features associated withthe illuminated periodic structures, or used to determine relativeregistration error between overlapping periodic structures.

The instrument includes one or more broad-spectrum light sources, e.g. axenon lamp and a deuterium lamp, and the light from such sources may besupplied to a movable measurement head via one or more optical fibers.Likewise, light collected by the measurement head optics may bedelivered to the spectrometer via one or more optical fibers. At leastone polarizing element (fixed or rotatable) is situated in the beampath, with preferably a polarizer in each of the illumination andcollection paths, and there may also be a polarization modulatingelement associated with any of the polarizers.

The measurement head may be rotated by a motor-driven mechanism toorient the plane of incidence (and collection) to different azimuthorientations θ relative to the sample. This concept of a measurementhead that can be rotated as a unit to different azimuth directions canbe extended to other related instruments that have a non-normal incidentbeam or other directional anisotropies in their optics, including forexample spectroscopic ellipsometers. In addition to employing non-normalincidence and collection, ellipsometers also include rotatingcompensators and analyzers which, like the polarimetric scatterometer,establish a specific polarization direction to the light. The ability torotate the measurement head allows measurements to be made on wafers atany arbitrary orientation.

The instrument, or at least the measurement head thereof, can beintegrated into a wafer processing tool and wafer samples delivered tothe instrument for measurement. The measurement head then moveslaterally over the wafer (or the wafer moves on a stage) to specificmeasurement spots. Spectral reflectance measurement at each spot is thenmade with the polarized light at preferably three or more differentazimuth angles by rotating the head. In some cases, it may be preferablefor the sake of simplicity to measure the spectral reflectance at oneazimuth angle, where the head is rotated so the plane of incidence onthe wafer is, e.g., perpendicular to the array of periodic structures onthe grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side plan view of a preferred embodiment of apolarimetric scatterometer of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIG. 1, the light box 1 has a high pressure xenon lamp2 to provide illumination between 220 and 1100 nm. It also has adeuterium lamp 3 to provide deep-UV light between 190 and 240 nm. Theshorter wavelength UV light helps give better results on smallerstructures. A mirror 4 can be moved into the beam path to select betweenthe two light sources. A shutter 5 can flip into the beam to cut off alllight. The shutter both protects the optics from excessive UV light andallows measurement of the detectors' background signal. The light boxoptics create an image 6 of the sources that is large enough toilluminate several multi-mode optical fibers 7, 19 and 22. One of thesefibers 7 preferably has a core diameter of 100 microns. The output endof fiber 7 is imaged onto the wafer 12 with a demagnification ofapproximately 2× using curved mirrors 9 and 10. The angle of incidenceon the wafer is non-normal, i.e. greater than 4° away from perpendicularto the wafer surface, and is typically anywhere between 5° and 50°. Afused silica window 11 protects the wafer 12 from particles generatedfrom the moving optics, and also protects the optics from outgassingfrom the wafer.

A polarizer 8 linearly polarizes the light that is incident onto thewafer. This polarization is selected to maximize the sensitivity of theoptical measurement to the parameters of the sample. In a preferredimplementation, polarizer 8 allows only the S-polarized light onto thesample. In a more complex version of the invention, the polarizer 8could be made to rotate to collect even more information about thesample. Also a rotating waveplate or electro-optic modulator (Pockels orKerr effect based) could be placed after polarizer 8 also to collectmore information on the sample's properties.

Mirrors 9 and 10 have their centers of curvature lying on or near a linebetween the output end of fiber 7 and its image on the wafer. Thesemirrors fold the light path so that the non-normal incidence does notlead to unnecessarily large footprint for the measurement head. This isdesired so that the head can be integrated within a wafer process tool.Mirror 9 is a first surface convex mirror, while 10 is a concave fusedsilica mangin mirror. The two mirrors 9 and 10 correct the chromatic andother aberrations introduced by the window 11 and the polarizer 8 sothat the image of the fiber end has near diffraction-limited qualityfrom 190 to 1100 nm. A prescription for a representation optical designis given in a appendix A. This prescription is optimized for aparticular angle of incidence, window thickness, working distance, etc.Modifications for other particular design configurations could be madeby one skilled in the art.

The light reflected from the wafer is collected and imaged by mirrors 13and 14 that are similar to mirrors 10 and 9, with their centers ofcurvature preferably on a line between the measurement spot on the wafersample 12 and the input to the optical fiber 16, and correcting for anyaberrations introduced by window 11 and polarizer 15. The light travelsthrough a beamsplitting crystal polarizer 15 preferably made from alphabarium borate. The beamsplitting polarizer 15 directs images of thewafer onto the input ends of fibers 16 and 18. Fiber 16 receivesS-polarized light while fiber 18 receives P-polarized light. The twofibers 16 and 18 have smaller core diameters than fiber 7 so that themeasurements are not overly sensitive to focus. The diameter of the endof fibers 18 and 16 determine the geometric size of the measurement spoton the wafer, typically less than 200 μm diameter. A less complexversion of this apparatus would have the beamsplitting polarizer 15replaced by a simple polarizer so that only one polarization isdetected. Another version would have polarizer 15 omitted, but thatversion would be less useful in measuring three-dimensional gratingstructures such as arrays of holes. More complex versions would replace15 with a rotating polarizer, or fixed polarizer with rotating waveplateor electro-optic modulator.

Fibers 16 and 18 carry the two polarizations of reflected light to amulti-channel spectrometer 17 that has the capability of measuring thespectrum from each fiber simultaneously. Spectrometer 17 would haveeither multiple linear detector arrays or a two dimensional detectorarray. Multiple standard spectrometers could also be used in place ofthe multi-channel spectrometer 17. Alternatively, the illumination ordetection could use a monochromator to serially perform measurements atvarious wavelengths of light.

Another fiber 19 exits the light box and closely follows the path offiber 7 until it enters the measurement head 30. At that point it iscoupled to another fiber 21 with a connector 20. Fiber 21 also closelyparallels fibers 16 and 18. Fiber 21 also enters spectrometer 17, whereits spectrum is used to correct for variations in the lamp intensity ortransmission of the fibers versus measurement head position.

Alternatively, fiber 21 could direct its light to a photodiode (notshown) if the intensity variations are not substantially spectrallydependent. Ideally the fibers are routed so that as the measurement head30 moves, the radius of any bends in the fiber remain constant.

The data from the spectrometer or photodiode preferably goes to aprocessor (not shown) where it is converted into determinations of theparameters of the structure on the wafer at the measurement spot. Forexample, the parameters might be the thicknesses of films, line width(critical dimension or CD), the sidewall slope of lines, etc. In oneembodiment, the data is converted into spectral, absolute, polarizedreflectance and compared to the spectrum library to find the best matchand therefore the unknown structure parameters. There are manyalternative approaches to process the data from the spectrometers toyield structure parameters. For example, the theoretical responses forcomparison to the processed spectrometer data may be calculated withmodel parameters determined after the data is obtained. This is indistinction to comparing to library responses which are precomputed forpredetermined parameter values and stored. In yet another embodiment,parameters of structure may be calculated directly from the spectrometerdata, i.e., without comparing to model results as an intermediate step.

A third fiber 22 exits the lamp house 1 and has its output end in theobject plane next to the end of fiber 7. The light from fiber 22 travelsthrough the optical system (elements 8-10 and 13-15), reflecting off thewafer and forming an image of the exit end of fiber 22 onto the entranceend of fiber 23. Fibers 22 and 23 are the same size so that the signalwhich is detected by a photodiode 24 is extremely sensitive to waferfocus. Autofocus is performed by scanning the measurement head 30 in Zand moving to the Z position that maximizes signal on photodiode 24. Thelight source for fiber 22 could also be a laser.

While the scatterometer system described above uses a remote lightsource and remote spectrometer (i.e., not mounted to the movablemeasurement head), and couples these elements to the measurement headoptics via a set of optical fibers, it is also possible to mount one orboth of these elements directly to the measurement head. The additionalmass of such a measurement head may slow the movement of the headsomewhat, but the head will still be usable, especially if the sourceand spectrometer are reasonably lightweight and if the motors used tomove the head assembly are reasonably powerful. An advantage of directmounting is that twisting of the optical fibers during azimuthalrotation of the measurement head can be avoided. Instead, the light fromthe source is directed to the sample and reflected light is collectedfrom the sample in the normal manner, i.e. with mirrors and lenses.

The measurement head 30 is mounted to a rotating mechanism (not shown)that allows it to rotate in azimuth around the vertical axis 32. In anintegrated wafer processing station, the head 30 might also be mountedto an X-Y mechanism (not shown) for providing lateral motion of the headso that it can measure different points on the wafer without having tomove the wafer. Alternatively, an X-Y stage could be provided to movethe wafer, especially for stand-alone measurement systems. In that case,X-Y lateral motion of the optical measurement head is unnecessary. Ifdesired, a reduced motion stage, such as ½X,½Y stage, with or withoutwafer stage θ rotation, or a polar coordinate (R, θ) stage, can be usedto reduce the footprint required by the wafer. Yet another configurationwould have the wafer placed on a rotary stage, and the rotatablemeasurement head 30 mounted for radial motion. Basic constructiondetails of rotation and X-Y drive mechanisms are well known in the artand are applicable here. The rotational axis of the measurement head 30should coincide with the symmetry axis 32 of the measurement optics andintersect the illuminating beam at the spot on the wafer sample 12.Alternatively, a controller may have information about the relativepositions of the two axes, and position the measurement head withrespect to the wafer accordingly. A complete 360° rotating range ispreferred, although a 180° range would also be acceptable in many cases.A driver motor can be used, coupled to a rotatable plate on which themeasurement head optics assembly is mounted. If needed, this entire unit(motor, plate, and optics) can then be mounted to the X-Y mechanism,along with the other elements within dashed box 31.

Collecting data at multiple azimuth angles on a grating structure on thewafer produces additional independent measurements that reveal moreinformation about the structure and allows either measurement of moreunknown parameters of the structure or more robust determination of afew unknowns. In the case where the wafer can not be rotatedarbitrarily, the measurement head azimuth rotation also allows the planeof incidence to be aligned in the preferred perpendicular directionrelative to the grating lines even when multiple azimuth angles are notneeded. Furthermore, the ability to measure at two azimuth angles 180°apart can be used to reduce the sensitivity of the measurements to wafertilt and other asymmetries in the optical system. As previously noted,this concept can be extended to related optical instruments, such asspectroscopic ellipsometers, that also have directional anisotropies intheir optics. Like the preferred polarimetric scatterometer shown in theFIGURE, ellipsometers also have non-normal incidence and collection of alight beam. They also have specific polarization directions establishedby rotatable compensators and analyzers. Being able to rotate an opticalmeasurement head in a range of azimuthal directions allows theinstrument to be used with wafers or other samples at arbitraryorientations, since the head can be rotated to the desired measurementorientation.

Everything within outline 31 is mounted to the X-Y stage. The elementsoutside 31 may optionally be located remotely where they do not occupythe limited space available inside a wafer process tool. Within 31 thereis also a viewing subsystem (elements 25-29) to view the wafer patternsand allow the exact position and orientation of the wafer to bedetermined relative to the X-Y stage or measurement head. A large LED 25provides the illumination. Preferably 25 is a two color LED so that thesecond color can be used in case the first does not provide sufficientcontrast. Alternatively, two LEDs of different colors with a dichroicbeamsplitter to combine both beams could replace the single LED 25. Aconventional visible beamsplitter 26 directs the illumination to anobjective 27. The LED 25 is preferably placed near the entrance (orupper) pupil of 27. There are many alternative sources of illuminationfor a viewing system. Objective 27 and tube lens 28 form an image of thewafer on a CCD video camera 29. The position of the center of the fieldof view of camera 29 on the wafer is measured and calibrated (it has aknown fixed offset distance) relative to the position of the image offiber 7 on the wafer. The image from the camera 29 is fed into a patternrecognition subsystem that determines the position and angle of thewafer pattern.

A second angle of incidence can be accommodated inside measurement head30 by having a second set of optics (not shown) with a different angleof incidence oriented in a plane separate from that of the first angleof incidence. This second angle of incidence can likewise be changed toother azimuth directions relative to the sample 12 by rotating themeasurement head 30.

What is claimed is:
 1. An apparatus for measuring the optical propertiesof a periodic structure on a sample, comprising: a movable measurementhead having optics directing a polarized illumination beam onto theperiodic structure on the sample and collecting light reflected from ameasurement spot of the periodic structure, wherein the movablemeasurement head is rotatable in azimuth relative to the sample, whereinthe measurement spot defined by the measurement head has a size lessthan 200 microns, and wherein the illumination beam directed by themeasurement head optics is incident on the sample at an angle greaterthan 4 degrees; and a near point-like light source, a tilted convexmirror, a tilted concave mangin mirror, said mirrors forming awell-corrected image with light focused through a window tilted withrespect to a chief ray, said mirrors having their centers of curvaturelocated on a line between the source and image.
 2. The apparatus ofclaim 1 where the image is corrected for wavelengths greater than 400nm.
 3. The apparatus of claim 1 where a polarizer lies between thesource and the convex mirror.
 4. An apparatus for measuring the opticalproperties of a periodic structure on a sample, comprising: a movablemeasurement head having optics directing a polarized illumination beamonto the periodic structure on the sample and collecting light reflectedfrom a measurement spot of the periodic structure, wherein the movablemeasurement head is rotatable in azimuth relative to the sample andwherein the measurement spot defined by the measurement head has a sizeless than 200 microns; and further including a separate fiber to monitortransmission changes in said polarized illumination beam.
 5. Anapparatus for measuring the optical properties of a periodic structureon a sample, comprising: a movable measurement head having opticsdirecting an illumination beam at an incidence angle greater than 4°onto the periodic structure on the sample and collecting light reflectedfrom a measurement spot of the periodic structure, wherein the movablemeasurement head is rotatable in azimuth relative to the sample whereinthe measurement spot defined by the measurement head has a size lessthan 200 microns; and a near point-like light source, a tilted convexmirror, a tilted concave mangin mirror, said mirrors forming awell-corrected image with light focused through a window tilted withrespect to a chief ray, said mirrors having their centers of curvaturelocated on a line between the source and image.
 6. The apparatus ofclaim 5 where the image is corrected for wavelengths greater than 400nm.
 7. An apparatus for measuring the optical properties of a periodicstructure on a sample, comprising: a movable measurement head havingoptics directing an illumination beam at an incidence angle greater than4° onto the periodic structure on the sample and collecting lightreflected from a measurement spot of the periodic structure, wherein themovable measurement head is rotatable in azimuth relative to the samplewherein the measurement spot defined by the measurement head has a sizeless than 200 microns; and further including a separate fiber to monitortransmission changes in said polarized illumination beam.
 8. Anapparatus for evaluating a periodic structure on a semiconductor wafer,comprising: a broadband light source for generating a probe beam; anoptical assembly for coupling the probe beam from the broadband lightsource to a spot on the surface of the semiconductor wafer including theperiodic structure, the optical assembly further collecting lightspecularly reflected from the sample; a spectrometer operable to measurethe collected specularly reflected light and generate an output signalas a function of wavelength; and a rotating mechanism coupled to theoptical assembly for rotating the optical assembly such that a planedefining the incident and specularly reflected light is rotated therebyvarying the direction in which the probe beam impinges upon the samplewherein the rotational axis of the optical assembly intersects with theprobe beam spot on the sample.
 9. An apparatus according to claim 8,wherein: the optical assembly is coupled to at least one of thebroadband light source and the spectrometer by at least one opticalfiber capable of carrying the probe beam.
 10. An apparatus according toclaim 8, wherein: the spectrometer is rigidly attached to the opticalassembly.
 11. An apparatus according to claim 8, wherein: the broadbandlight source is rigidly attached to the optical assembly.
 12. Anapparatus according to claim 8, wherein: the spectrometer is furtheroperable to measure characteristics of the periodic structure selectedfrom the group consisting of width, profile, and thickness.